“Mommy, when will it be 2007 again?” — What is time, and how does it work?

Back in 2013, when my friend Sophia was six years old, she asked, “Mommy, when will it be 2007 again?” She had been talking with her mom about small parts of a second, like picoseconds and nanoseconds. So her mom asked me the question. Here is my answer:

Hello dear Sophia!

Your mom says you asked, “Mommy, when will it be 2007 again?” What a great question! I spoke with one of the people I work with at NASA, a very nice, very smart man named Dale Fixsen. We talked for about half an hour about what time is like, and if it has a direction, and if it repeats itself.

Here is what we know. All of the stuff that there is, including you and me and your sisters and parents and friends and all of your toys and all of everybody and everything, just keeps getting more complicated. This is because you know more people and have done more things, and so you have more memories of those people and those things. You store memories in your brain. The more memories you have, the more complicated your brain is. All the things you experience get more and more complicated, and so your brain gets more and more complicated to store all the memories of those things. That is how you can tell what direction time goes. If you can remember it, then it must have happened in the past.

It’s not just people that work like this. If you have a box of cookies, and you shake it really hard, then some of the cookies will break. So the cookies have a kind of memory of the shaking they experienced, because they are in more pieces now, so the stuff in the box is more complicated than it was before. You can’t ever shake a box full of broken cookies and expect the cookies to unbreak and become whole cookies again.

So, if somebody or something has some kind of memory of 2007, either because they can remember it, or just because the cookies from 2007 have been eaten and digested and pooed out and flushed down the toilet, then 2007 can never happen again. In fact, any moment can’t happen again, because each moment in time causes the stuff everywhere to get a little more complicated.

“Mommy, when will it be 2007 again?” It will not be 2007 again. Each moment is unique, there is no other moment that is the same. Each moment is special. Enjoy as many as you can!